Poster presented to the Biomedical Engineering Society Conference, Chicago, IL 10/2006 ABSTRACT Assembly Of Trabecular Bone Derived Scaffold Micro-Architectures For Patient Specific Applications M. Wettergreen, B. Bucklen And M. Liebschner Rice University, Houston, TX To improve upon the development of scaffolds for orthopedic tissue engineering purposes, we developed a library of architectures (unit primitives) that may be strategically merged according to patient specific design constraints. In particular, for bone, mechanical characteristics such as the regional stiffness in a continuum sense, micro-architectural levels of mechanical surface strain, void fraction amount and orientation, as well as permeability are critical both individually and in concert. As the influences of these factors are elucidated, the potential to successfully engineer scaffolds improves. Here we expound upon previous research of creating assembled scaffolds from derived analytical shapes, and extend it to encompass the native architecture of human trabecular bone. Architectures were derived from repeated patterns witnessed in the interior portion of various cadaveric T-9 vertebral bodies. We introduce a library in which the apparent properties, independent of tissue material properties, may be matched in a patient/site specific manner, yet the architecture maintains much of the same tissue level shape (porosity, permeability) that are essential for its biological functionality. Multiple architectural configurations with similar elastic properties are possible via the joining of the architectures using a characterized set of interfaces resulting in a global assembly constructed from a regional bone density map.

1. ASSEMBLY OF TRABECULAR BONE DERIVED SCAFFOLD MICRO-ARCHITECTURES FOR PATIENT SPECIFIC APPLICATIONS *Wettergreen, M A; *Bucklen, B S; *Liebschner, M A K *Dept. of Bioengineering, Rice University, Houston, TX, [email_address] INTRODUCTION Orthopedic tissue engineering incorporates the use of scaffolds which act as a temporary conduit for cell attachment and tissue growth. The clinical need arises in the areas of tissue degeneration due to osteoporosis, voids caused by tumor resection, and a variety of other genetic diseases and trauma affecting mature bone. When autografts and allografts are non-ideal, tissue scaffolds must be designed. Unfortunately, the interplay of factors thought to be critical in the process (such as stiffness, strength, permeability, surface-volume ratio, mechanical surface environment) is virtually unknown. At the very least, the design must incorporate the apparent mechanical properties of the trabecular bone we are trying to replace. Here, we discuss a framework for the design of an implant from design to manufacture. FUTURE OF COMPUTER-AIDED TISSUE ENGINEERING TISSUE MODELING: SELECTION, OPTIMIZATION, AND ASSEMBLY Figure 2. Optimization of Surface Mechanical Properties. A heuristic voxel-based procedure was used to improve the surface stress environment of initial unit cells toward a uniform stress state. Figure 3. Unit Cell Characterization. Finite Element Modeling (FEM) helped characterize tissue primitives at their average native porosity. Figure 1. Tissue Modeling (Identification of Tissue Primitives). Repeated shapes witnessed in the trabecular portion of human T-9 vertebral bodies were documented and transferred into CAD models. Figure 4. Unit Scaffold Assembly. The vertebral body is scanned, density values of subregions corresponding to the unit cell size are ranked. Unit Cells are placed according to the density map, and interfaces from an interface library are selected to match adjacent faces of the unit cells. The selection is determined by a system which indexes the unit-cell – interface intersection volume and interface – interface intersection surface area. Computer-Aided Tissue Engineering. Patient-specific CT data is used to develop tissue models which replicate the apparent properties of trabecular bone (i.e. modulus, permeability, etc.). Models are manufactured, validated ex vivo , then implanted Figure 6. Mechanical Usage and Implant/Tissue Degradation Profiles. The mechanical usage window (left) hypothesizes that metabolic changes in bone mass occur with strain at the micro-architectural level. Large strains result in an irreversible, pathological overload and tissue necrosis. Tissue engineered implants must provide initial mechanical support. Yet, as the scaffold degrades and new tissue is formed, increased strain nearing the pathological window may be induced (right) . Depending on the scaffold architecture, regional differences exist. Implant design must take this into account. MANUFACTURE / FABRICATION Figure 5. Fabrication of CATE Library into a Global Construct. Assembly of a partial, human vertebral body (top, left) is translated for 3-D printing via a fused deposition rapid prototyping system which uses a thermoplastic wax material (top, right) . Other methods of fabrication include selective laser sintering, which cures a powder polymer in a layer-by-layer fashion by heating the material to just above its glass transition temperature (bottom, right). Acknowledgements: Chris Chen, Jeremy Lemoine, Stefan Lohfeld. Funding provided by NSF.